In reviewing Diane Ravitch's book a few weeks ago, I pointed out that it took decades for baseball statistics to mature, and that statistics measuring teaching effectiveness would likely take awhile, too. Here's the abstract of a clever study from Science:

Children’s reading achievement is influenced by genetics as well as by family and school environments. The importance of teacher quality as a specific school environmental influence on reading achievement is unknown. We studied first- and second-grade students in Florida from schools representing diverse environments. Comparison of monozygotic and dizygotic twins, differentiating genetic similarities of 100% and 50%, provided an estimate of genetic variance in reading achievement. Teacher quality was measured by how much reading gain the non-twin classmates achieved. The magnitude of genetic variance associated with twins’ oral reading fluency increased as the quality of their teacher increased. In circumstances where the teachers are all excellent, the variability in student reading achievement may appear to be largely due to genetics. However, poor teaching impedes the ability of children to reach their potential.

SEATTLE (AP) — Genetics play the biggest role in determining how fast a child learns to read, but a good teacher can make a measurable difference as well, according to a study released Thursday.

Florida State University used twins assigned to different classrooms to develop the conclusions.

Researchers studied more than 550 first- and second-grade classrooms with at least one identical twin and more than 1,000 classes with at least one fraternal twin.

Among the identical twins, 42 pairs out of 280 pairs showed significant differences in reading improvement during the year studied, said lead researcher Jeanette Taylor, an associate professor of psychology at Florida State.

In each case, the teachers also had significantly different quality scores. Twins with similarly good teachers got similar scores.

"If you have identical twins, they should do very similarly in school," Taylor said.

Teachers whose students showed the greatest average one-year improvement in the number of words they could read out loud in one minute were considered the best teachers for the purpose of the study. ...

The study shows teacher quality is one reason for the differences between the achievements of twins, but it cannot explain the whole difference, Taylor said. Since these pairs were all in two different classrooms, other possible factors include how well behaved classmates are.

The researchers believe their results showed the best teachers made the biggest difference in learning achievement. Genetic differences between students seemed to disappear in classrooms taught by less effective teachers, because children don't reach their potential, the researchers found. ...

"Good classrooms are all alike; they maximize kids' potential. Poor classrooms are not only poor in one way; they are poor in multiple ways," Shanahan said.